The present invention relates to a method and apparatus for testing the pressure in subterranean wellbores. More particularly, the present invention relates to a method and apparatus for testing the pressure in producing subterranean wellbores. In a still more specific aspect, the present invention relates to a method and apparatus for testing the presence in a producing, subterranean geothermal wellbore.
The recovery of fluid from subterranean reservoirs involves a wide variety of techniques. Petroleum is most often produced by depending upon the fact that the fluid is contained in the reservoir under high pressure, and, when the reservoir is penetrated by a wellbore and the pressure released, the petroleum will be driven to the surface of the earth for recovery by natural pressure, such as a bottom water drive, solution gas drive and the like. To the extent that reservoir pressure is insufficient to drive the oil to the surface or the reservoir has been depleted to the extent that the pressure is insufficient, the well will normally be put on pump or vacuum to produce further amounts of oil. All these techniques are generally referred to as "primary" recovery techiques. Once the reservoir has reached its limits of production by such primary recovery techniques, it is necessary to supplement the natural reservoir pressures or in some other manner drive the oil from the reservoir. For example, water flooding as well as the injection of various gases at low pressure are resorted to to displace oil from the reservoir. While these techniques do recover additional amounts of oil, they have proven to be highly inefficient, thus giving rise to the development of more sophisticated techniques of recovery such as the injection of gas at high pressure to create a zone of miscibility between the drive gas and the oil, the injection of a small slug of propane or the like followed by a gas to create a miscible displacement at lower pressures, the injection of a variety of oil-miscible solvents followed by a gas or, in the case of highly viscous oils, the passage of a flame front through the reservoir. All of these techniques are commonly referred to as "secondary" recovery techniques. Following the practice of a secondary recovery technique, such as water flooding, still further recovery of oil can be attained by the so-called "tertiary" recovery techniques. These techniques generally involve the injection of a solvent, such as a surfactant, or a polymeric material which id mutually miscible with oil and water and driving the slug through the reservoir by water. In some instances, it is also common, when the oil is highly viscous, to utilize a viscous polymeric material to improve recovery.
Because of the decline of oil reserves, interest in geothermal reservoirs, such as hot water and stream reservoirs, as a source of energy has increased significantly. In the production of fluids from geothermal reservoirs, the inherent high pressure of the reservoir, in its natural state, is again depended upon to produce fluids from the reservoir and essentially the same primary recovery techniques employed in oil recovery are utilized.
Obviously the most important factor in a production of fluids from subterranean reservoirs is timely and accurate information concerning the pressure of the fluids in the reservoir. While some information can be obtained from pressures at the surface of the well bore, the most useful information is obtained by measuring the pressure adjacent the production formation. Further, while wellbore pressure information under static or shut-in conditions is valuable, the most valuable information is obtained by pressure measurements obtained while the well is on production. For example, reservoir potential can be predicted by using pressure drawndown and buildup data in a producing well and the productivity of the formation can be evaluated in a producing well. The reservoir characteristics and fluid properties obtained from pressure data can be utilized in numerous other ways, for example, pressure information facilitates the design of surface and subsurface producing equipment. Detailed pressure information prior to production is also helpful in determining the best rate of production and estimating the life of the reservoir. In addition, producing and injection well pressures are necessary and in the design and operation of secondary and tertiary recovery techniques.
Numerous methods are available for determining pressures at various depths in a wellbore. In general, an automatically recording, pressure-sensitive sonde may be introduced into the well by means of a wire line and then withdrawn and the recorded pressures observed. In a similar technique, a pressure sensitive sonde or sub can be lowered into the well on a wire line and the pressure data transmitted to the surface through an electrical line. These wire line techniques have the advantage that they can be run in a producing well, through a well control device, such as a stuffer box. However, instruments of this variety are not only expensive but are quite delicate and will generally be incapable of withstanding adverse environmental conditions such as high temperatures, high producing rates and high pressures. It is also known to lower a pressure sensitive element to the desired depth in the well and transmit the pressure information to the surface through a column of fluid of known density. Here again, the pressure sensitive equipment is not adapted to use in harsh environments and the accuracy of the measurement is open to some question. It is also possible to lower a device into the well to take a sample of subsurface fluid and retrieve the sample chamber with the fluid under the pressure at which the sample was taken. In this technique the lack of adaptability to harsh conditions and accuracy are also problems. Finally, techniques have been developed for balancing the well pressure at a given depth against a column of gas. While this technique has numerous advantages over those previously discussed, due to its relative simplicity, numerous problems are also inherent. For example, the placement of the equipment in the reservoir is a problem, the equipment generally lacks flexibility in adapting the same to wide pressure variations and the equipment lacks adaptability to severe conditions of high temperatures, flow rates and pressures. The most common technique of utilizing this method involves attaching the equipment to the bottom of a conventional string of tubing as a sub or sonde and running the small open-ended tubing, for carrying the pressure transmitting gas, along the outside of the conventional tubing. In the last instance, the techniques for utilizing the equipment contribute to the difficulty of placing the equipment in the well, to the extent that the pressure transmitting gas in the small or microtubing is introduced until all reservoir fluid is displaced from the open ended tubing and an indication of complete displacement is dependent upon the gas bubbling out the bottom of the tubing. The latter is generally observed by a flattening of the pressure curve or an essentially constant pressure observation. Here again the accuracy in determining this point becomes questionable in many cases. Finally, most of the pressure balancing type of pressure measuring equipment as well as the other types, aside from the wire line type, are mounted in the reservoir as a permanent installation. While this has advantages of providing pressure information throughout the life of the reservoir, it is disadvantageous to the extent that it must be installed prior to initiation of production of the well or the well must be shut down and often times of equipment, such as certain parts of pumping equipment, must be removed temporarily and then replaced after the pressure sensing equipment is placed.
The previously mentioned disadvantages of conventional pressure testing equipment are exaggerated even more when such devices are to be utilized in geothermal wells. In these cases, the harsh conditions of high temperatures, high flow rates and high pressures make running, and use of the equipment difficult and the dependability highly questionable.
It is therefore an object of the present invention to provide an improved method and apparatus for pressure testing in a subterranean wellbore which overcomes the above mentioned problems of the prior art. Another object of the present invention is to provide an improved pressure testing method and apparatus which can be utilized under adverse environmental conditions of high temperatures, high pressures and/or high flow rates. Yet another object of the present invention is to provide an improved method and apparatus for testing wellbore pressures which produces highly accurate results. Another and further object of the present invention is to provide an improved method and apparatus for pressure testing in a wellbore which requires no specialized components and is economical in construction and use. Another object of the present invention is to provide an improved method and apparatus for pressure testing in a wellbore which can be utilized on a temporary or permanent basis. Still another object of the present invention is to provide an improved pressure testing method and apparatus for a wellbore which can be conveniently introduced into a producing well through conventional well control equipment. Yet another object of the present invention is to provide an improved pressure testing method and apparatus for a wellbore which can be introduced into the well without interference with other well equipment and/or being interfered with by other well equipment. Another and further object of the present invention is to provide an improved method and apparatus for pressure testing in a wellbore which is capable of simple modification to permit it adaptation to a variety of different conditions, particularly large pressure fluctuations. Yet another object of the present invention is to provide an improved method and apparatus for testing pressure in a wellbore by a pressure balancing technique in which the apparatus can be introduced into the well in a sealed condition, placed in operation in a simple manner and leaves no doubt as to the time at which pressure measurements can be initiated. These and other objects of the present invention will become apparent from the following description.